Field of the Invention
The present invention relates to a semiconductor device, and particularly to a high-withstand-voltage semiconductor device including a diode and for use in electric-power applications.
Description of the Background Art
In these years, inverters are used in those fields such as the field of industrial power units. For the inverter, usually a commercial power source (AC power source) is used. Thus, the inverter includes a converter unit first converting an AC voltage into a DC voltage (forward conversion), a smoothing circuit unit and an inverter unit converting the DC voltage into an AC voltage (inverse conversion). As a main power device of the inverter unit, an insulated gate bipolar transistor (hereinafter referred to as “IGBT”) capable of performing switching operation at a relatively high speed is chiefly employed.
In most cases, the load of the inverter is an electric induction machine (motor which is an inductive load). The inductive load is connected to a point of an intermediate potential between an upper arm element and a lower arm element, and electric current flows to the inductive load in both of the positive and negative directions. Therefore, in order to direct the current flowing in the inductive load from the end where the load is connected back to the power supply of a high potential and to direct the current from the end where the load is connected to the ground, a freewheel diode for circulating the current between the inductive load and the closed circuit of the arm elements is necessary.
In the inverter, usually the IGBT is operated as a switch to repeat the OFF state and the ON state so as to control the power energy. Regarding the switching of the inverter circuit with an inductive load, the ON state is reached through a turn-on process while the OFF state is reached through a turn-off process. Here, the turn-on process refers to a change of the IGBT from the OFF state to the ON state while the turn-off process refers to a change of the IGBT from the ON state to the OFF state. While the IGBT is in the ON state, current does not flow through the diode and the diode is in the OFF state. In contrast, while the IGBT is in the OFF state, current flows through the diode and the diode is in the ON state.
A structure and an operation of a conventional diode will be described. In the conventional diode, a p-type diffusion region to serve as an anode is formed on one main surface side of an n-type low-concentration semiconductor substrate. On the p-type diffusion region, an anode electrode is formed such that the anode electrode contacts the p-type diffusion region. On the other main surface side of the semiconductor substrate, an n-type ultrahigh-concentration impurity layer is formed as the topmost surface. Under the n-type ultrahigh-concentration impurity layer, an n-type high-concentration impurity layer is formed. On the n-type ultrahigh-concentration impurity layer, a cathode electrode is formed such that the cathode electrode contacts the n-type ultrahigh-concentration impurity layer.
In order to ensure a withstand voltage of the diode in the state where a voltage is applied between the cathode electrode and the anode electrode, the diode including a guard ring (p-type layer) is commonly and widely used. The guard ring is formed to surround the anode at a distance from an end of the anode (p-type diffusion region), so that the electric field on an outer peripheral end portion of the p-type diffusion region is alleviated.
In an ON state where a high voltage is applied in the forward direction between the anode and the cathode, a large number of carriers are accumulated in a first-conductivity-type region (drift layer) of the semiconductor substrate. In contrast, in an OFF state where a high voltage is applied in the reverse direction between the anode and the cathode (at the time of reverse recovery), the carriers accumulated in the drift layer are discharged so that a reverse recovery current flows. At this time, a large current and a large voltage are applied to the diode, and accordingly heat is generated which is accompanied by large power consumption. This is one of the causes of hindrance to fast switching.
Japanese Patent Laying-Open Nos. 2003-152197 and 09-246570 for example disclose a diode including a guard ring.
The conventional semiconductor device has the following problem. In the ON state of the diode, carriers are diffused and accumulated not only in a region of the drift layer that is located immediately under the anode but also a region of the drift layer that is located immediately under the guard ring.
In the OFF state, carriers accumulated in the drift layer are discharged from the anode or cathode, for example, and disappear in the end. At this time, into the p-type diffusion region of the anode, both of carriers (holes) accumulated in the region of the drift layer immediately under the anode and carriers (holes) accumulated in the region of the drift layer immediately under the guard ring flow. Therefore, particularly in an outer peripheral end portion of the anode located in close proximity to the guard ring, current concentration occurs. Further, in the outer peripheral end portion of the anode, the electric field is intense at the time of reverse bias. Thus, the electric current and the electric field act on the outer peripheral end portion of the anode, and the outer peripheral end portion is a portion that is most likely to be broken in a marginal test.